Most moves can easily be done w/a DB, and I see no real advantage of the KB in things like pressing and squatting. When doing one arm work like snatching or pressing, you're going to get an offset effect even w/a DB. However, the KB makes it much easier to do some of the conditioning work, like figure eights, tactical lunges simply because the handle is not inside the weight plates, and easier to grab. Also real easy to switch hands, not to mention it is a thicker grip, and some of the MMA types prefer that. Also, when doing swings, I just find them much more accomodating. Personally, I use DB's for the major basics, and do my swinging, and repition snatches, figure eights etc w/ the KB. I think it boils down to personal preference.
Tim

theres not much of a difference buuuutttt the handle on the kettle bell allows for balance since the kettlebell can rest on ur forearm as you hold the handle which isnt that much of an advantage but you can do movements like "turkish get ups" much mroe smoothly. thats the only movement Id prefer that for

I use kettlebells for a lot of exercises that I used to use dumbbells for. TGUs and swings feel better with kettlebells. Even presses feel more balanced. I still prefer dumbbells for snatches since I'm still not comfortable with the catch with the kettlebell. With a dumbbell, there is no catch.

I've been playing around with kettlebells a little bit lately and really like them for Swings, Turkish Get Ups, One-arm Cleans, One-arm Overhead Squats, etc. Also, a pair of kettlebells could also be really good for Thrusters, but I still prefer a dumbbell for One-arm Snatches.